For those who accept or allow for a model in which Adam is the federal
head of a group of first humans, or some similar idea, how do you
interpret Paul's statement in Acts 17:26: "He made from one man every
nation of mankind to live on all the face of the earth, having
determined their appointed times and the boundaries of their
habitation..."?

I'm not sure I qualify to be a "polygenist" or not, I'd have to get at
look at Adam's tummy to be sure, but since I do advocate a model where
Adam is the federal head of the human race, I'll offer my comment.
First of all, Paul was in Athens speaking to Greeks who were Ionians -
distant descendants of Javan. Javan, a Japhethite, (Javana in Sanscrit)
is credited with founding the Greeks, sons of the Javanites referred to
in Joel 3:8. In the Iliad, Homer named Iawones (Hebrew Iawan) as the
progenitor of the Ionians known to the Hebrews as the Jevanim (Iewanim).
Had Paul been speaking to Romans I doubt he would have used these words
to that group.

"Every nation" in Hebrew-speak can be taken as many nations. We have
numerous examples in Scripture where the writers used "all" and "every"
just as we would say "much," "many" or "some." The "one man" likely is
Adam and the nations are those who were descendants of Ham, Japheth and
Shem. After a period of reconstitution after the flood the individual
tribes went out to settle their apportioned lands. At the death of Noah
in the days of Peleg was the earth divided among them (Gen. 10:25), and
off they went to settle in their appointed lands as detailed in that
same chapter of Genesis. These boundaries of habitation were important
because the cursed Canaanites took a land not apportioned to them which
Abraham had to take away. We know that Paul is speaking of God's chosen
race, not mankind in general, as he says in the next verse, "That they
should seek the Lord ."

If one wished to apply this verse to mankind in general, generic
humanity, who would that one man have been and what boundaries would
Paul be talking about? Certainly when man journeyed north out of Africa
some 60,000 years ago there were no boundaries, they covered the entire
earth.